Nonlinear response analysis of SDOF systems subjected to doublet earthquake ground motions: A case study on 2012 Varzaghan–Ahar events

• Time–frequency analysis of doublet earthquakes based on wavelet transform.
• Examine the validity of empirical relations of response parameters (e.g. strength reduction factors, or inelastic displacement factors).
• It is not appropriate to repeat the mainshock with the same time-frequency characteristics to produce multiple artificial seismic sequences.

A moderate earthquake ground motion with moment magnitude (Mw) 6.4 took place in the North-Western area of Iran on August 11, 2012, which was followed by an aftershock (Mw = 4.9) just 6 min later. Eleven minutes after the first event, the affected area was struck by another moderate seismic event (Mw = 6.3) coming from a different seismogenic source. Earthquake ground motions of this special type of seismic sequence were recorded in the cities of Varzaghan and Ahar. Therefore, the first part of this study investigated the energy distribution and frequency content characteristics of the Varzaghan–Ahar earthquake ground motions records through wavelet transform analysis. The second part of this study presents the results of the nonlinear response of SDOF systems subjected to as-recorded Varzaghan–Ahar seismic sequences. The nonlinear response was measured in terms of constant-ductility strength reduction factors, R, and inelastic displacement ratios, IDR, which were compared with predictive capabilities of previously proposed predictive expressions that take into account seismic sequences. Results indicate that the predictive expressions for R and IDR underestimate values obtained in this study for the intermediate- and long-period spectral region.